Electrodeposition as a coating application method involves the deposition onto a conductive substrate of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has gained popularity in the coatings industry because it provides higher paint utilization, outstanding corrosion resistance, and low environmental contamination as compared with non-electrophoretic coating methods. Both cationic and anionic electrodeposition is used commercially, with cationic being more prevalent in applications desiring a high level of corrosion protection.
Cationic electrodepositable compositions often comprise an aqueous resinous dispersion comprising (i) an active-hydrogen, cationic salt group containing film-forming resin, and (ii) an at least partially blocked isocyanate crosslinking agent. Such compositions also often include a catalyst for the reaction between the resin and the crosslinking agent, such as organotin compounds, among others. More recently, alternative catalysts, such as zinc (II) amidine complexes, have been introduced. Such catalysts are thought to provide better cure rates at relatively low temperatures (low temperature cures may be desirable to, for example, reduce energy costs) and may be less toxic and environmentally undesirable than, for example, organotin compounds.
One drawback to the use of zinc (II) amidine complexes as a catalyst in low temperature cure compositions, however, has been the inability to achieve stable aqueous dispersions comprising such catalysts in combination with an active-hydrogen, cationic salt group containing film-forming resin and an at least partially blocked isocyanate that deblocks at low temperatures. As a result, the use of such catalysts in “low temperature cure” applications has been difficult. The present invention, however, provides methods for using such catalysts in such applications.